Air Temperature and Precipitation

This section focuses on local and regional air temperature and precipitation patterns, and compares these values to historical conditions. Last winter was generally cooler than average (as is common during La Nina seasons), and saw near-normal precipitation and snowpack levels. Conditions changed abruptly near the end of April. The spring and summer months were warm (or hot) and exceptionally dry. The weather pattern changed to cool and damp for the last week of September, but we’ll have to wait and see if this change is lasting or fleeting.

The maps below show temperature anomalies and percent of normal precipitation for western Canada in September. Almost all of western Canada was warmer and drier than normal this month; however, the BC Coast saw temperatures closer to normal with much cooler temperatures during the last week of the month. The local temperature anomalies are not captured as well in this product, so we’ll look a little more closely at watershed conditions below.

Fig. 6 Temperature Anomalies for Western Canada in September

Fig. 7 Percent of Normal Precipitation for Western Canada in September

The table below shows monthly average air temperature and precipitation data for the Lower Capilano Watershed. May is certainly a standout month with average temperatures almost 4 degrees warmer than normal (and only 19.5 mm of rain), but temperatures have consistently been warmer than normal since late April. The watersheds received half as much rain as normal for the 5-month period of May to October.

Month	Mean Temp (°C )	Temp Anomaly (°C )	Precip (mm)	Precip % Normal
January	3.5	+0.4	318	67
February	1.8	-1.0	333	145
March	4.2	-0.6	118	35
April	6.2	-1.2	315	160
May	15.4	+3.7	19.5	16
June	16.5	+1.3	41	53
July	19.4	+1.8	44	73
August	18.9	+1.2	55	75
September	14.5	+0.5	100	56

Air Temperatures

The plot below shows the average daily temperature for 2023 (red line) compared to normals (gray shaded area) and the range of max and min (blue shaded area). New high and low temperature records are also plotted with red and blue dots. The sudden change in temperatures at the end of April is very clear. Nineteen new daily high temperature records have been set in 2023 so far, with nine high temperature records set in May alone.

Hottest Day of the year

The hottest recorded temperature this summer was on August 13th. The daytime high reached 35.3 degrees in the Coquitlam Watershed

Precipitation

The plot below shows the accumulated precipitation in the Lower Capilano Watershed in 2023. Last year is also shown in dark blue, and other years between 2003 and 2021 are shown with lighter shades of blue. Average annual accumulated precipitation is shown with the black dashed line.

The winter was marked by near normal precipitation, with the heaviest rain in February and the first half of April. It has been exceptionally dry since late April. Between April 24 and September 30, only 260 mm of rain fell in the lower Capilano Watershed, which is less than 50% of normal for this period.

We’ll take a closer look at drought in the next section.

Perhaps a better way to look at precipitation totals is to use the water year, which runs from October 1 to September 30. This captures the precipitation that falls in the autumn and early winter. As you can see below, 2023 (red) was a very dry year. 2022 was exceptionally wet despite dry conditions at the end of the water year. This was due to very heavy precipitation in the fall and early winter. 2015 is also highlighted to show how important snow is to our water supply. This year was actually slightly wetter than average for most of the year, and finished as one of the wettest of the past 20 years. It was a lack of seasonal snow that made 2015 such a challenging year from a water supply perspective.

It’s interesting that the selected analog comparison year of 2009 is the driest water year on record.

Dry Spells!

33 days (May 7 - June 9), and 25 days (June 23 - July 18)

Note: Only 0.5 mm of rain fell in 32 days between June 22 and July 24

Standardized Precipitation Evapotranspiration Index

The Standardized Precipitation Evapotranspiration Index (SPEI) is a multi-scalar drought monitoring index based on climatic data. It’s useful in determining the onset, duration, and magnitude of drought conditions, and captures the main impact of increased temperatures on water demand.

For this report, the SPEI is calculated over 3-month intervals. This is useful for analyzing seasonal drought in the Lower Mainland. Values that fall between 1.0 and -1.0 are considered normal, values below -1.0 highlight dry conditions (red), and values above 1.0 are wet (blue). Each bar in the plot below represents the SPEI value for the previous 3 months. As we can see, conditions have been dry-to-very dry for the past two summers. We’ll examine this drought more closely in the next section.

Note, the SPEI chart has not been updated for Oct 1.